There is an urgent need for a systematic study of the nature, range, and especially the consequences of individual differences in auditory abilities, in adults and children. Recent studies of the hearing of complex sounds have revealed as remarkable range of individual differences in auditory abilities. Assumptions about the significance of these differential abilities have influenced diagnostic practices and have justified widely marketed commercial therapy systems for the correction of auditory perceptual deficits. Support is requested for comprehensive studies of the auditory abilities of 800 adult listeners with normal pure-tone sensitivity, and for the auditory component of an ongoing cross-disciplinary study of 460 children in the first four grades of school (the Benton-IU Project). The research with adult listeners will employ an improved and extended version of the Test of Basic Auditory Capabilities (TBAC), plus a range of additional speech and nonspeech tasks. The research with children is a collaboration between optometrists, psychologists, and specialists in child language development, using the most comprehensive set of measures of sensory, cognitive, and linguistic abilities ever employed in an epidemiological/longitudinal study. Results of preliminary studies, assure that this research will provide valid and extremely cost-efficient answers to the basic questions: What are the major dimensions of auditory abilities?, and What are the consequences of deficits in those abilities for speech perception, language learning and reading? If warranted by the data collected with this test battery , a one-hour screening test will be developed to identify children with significant auditory perceptual deficits. Because the project is certain to identify subgroups of listeners with both unusually acute and unusually poor auditory abilities, selected subgroups of listeners will be further investigated using psychophysical and physiological measures, including additional spectral and temporal tests, extended training, early, middle and late evoked responses, cochlear emissions, and PET (and possibly FMRI) images obatined during auditory processing.